The present invention relates to an improved brake monitoring system particularly for use on heavy vehicles, such as a tractor and trailer combination, buses and the like, having a plurality of spring brake actuators.
Heavy-duty trucks, buses and other large vehicles are typically equipped with a pneumatic brake actuating system. The brake actuating system typically applies air under pressure to a service chamber of a brake actuator to move a diaphragm in a first direction. A push rod moves with the diaphragm and the push rod is connected to a linkage which actuates the vehicle brakes. In a piston-type brake actuator, the piston plate seals against the internal surface of the housing, eliminating the requirement for a diaphragm. An emergency chamber having a power spring and a second diaphragm is typically mounted on the service chamber and is operable to move the push rod and actuate the brakes in the event that the pneumatic vehicle system fails or when the vehicle is turned off when the vehicle is parked. Thus, the emergency chamber serves as an emergency braking system for the vehicle and a parking brake.
A brake actuator has a predetermined amount of available movement or stroke of the push rod. The amount of movement of the push rod required to fully actuate the braking system of the vehicle should be carefully monitored, such that it is within the available stroke of the push rod of the brake actuator. Excessive movement of the push rod can be created by one of several factors. Typically, excessive movement of the push rod is due to brake lining wear. As the brakes wear, more movement of the push rod is required to actuate the brakes. Further, if the linkages and connections between the push rod and the linkages, et cetera, bend or become loose or excessively worn, additional push rod movement may be required to actuate the brakes. A combination of these several factors may sometimes cause the amount of push rod movement required to actuate the brakes approach the available push rod movement or stroke available from the brake actuator. As will be understood, this is an undesirable situation.
The prior art has proposed various methods and apparatus to monitor the push rod movement during actuation of the brake and provide some indication to an operator as to when there is excessive push rod movement, which is referred to as xe2x80x9coverstroke.xe2x80x9d As will be understood, a determination of when there is excessive push rod movement or overstroke is dependent upon the designed or rated stroke of the brake actuator. For example, the push rod of a typical brake actuator may include a brightly colored ring, which may be painted on the push rod, indicating an overstroke condition when the ring extends out of the brake actuator during actuation of the brakes. The ring may, however, be difficult to see because of the location of the brake actuators beneath the truck or trailer and accumulated road debris. Automatic slack adjusters located between the push rod and the foundation brake are also conventionally used, wherein the slack adjuster incrementally adjusts to compensate for slack in the braking system and to decrease the required push rod movement.
The prior art has also proposed various electronic monitoring systems which generally monitor either the stroke of the push rod or the movement of the linkages between the push rod and the foundation brake including the slack adjuster. However, there are several obstacles to be overcome. First, powering and monitoring electronic indicators on each brake actuator of an 18-wheel vehicle is costly. Further, the hostile environment in which the brake actuators are mounted beneath the vehicle can damage the monitoring system, particularly where there are exposed pistons, sleeves, sensors, et cetera. Finally, where the stroke of the push rod is monitored by the brake monitoring system, it is essential that the push rod stroke monitoring system be accurately assembled on the brake actuator and be able to withstand the hostile environment of the brake actuator. Finally, it is desirable that the components of the brake monitoring system be easily and accurately assembled on the brake actuator preferably without special tools.
The present invention relates to an improved brake monitoring system which may include a plurality of brake monitors mounted on each of the plurality of brake actuators of a vehicle. As set forth above, a conventional brake actuator includes a housing having an opening therethrough, a reciprocal piston rod or push rod extending through the housing opening and typically an annular stone shield mounted within the housing opening surrounding the push rod preventing debris from entering the brake actuator housing.
As set forth above, the improved vehicle brake monitor of this invention may be utilized with any brake actuator which includes a housing and a piston or push rod reciprocable through an opening in the housing for actuation of a vehicle brake. A typical brake actuator includes a flexible cup-shaped diaphragm having a peripheral edge portion which is supported within the housing and a piston having a piston plate which is spring biased against the diaphragm by a return spring. The piston rod or push rod is reciprocable through an opening in the housing having a free end typically connected to a slack adjuster, which is connected to the foundation brake of the vehicle. Upon actuation of the brake by the vehicle operator, the pneumatic pressure of the vehicle is received by the brake actuator, inverting the flexible diaphragm and driving the piston rod through the opening in the brake actuator housing, actuating the braking system of the vehicle. As will be understood, however, the improved brake monitor of this invention may be utilized with any brake actuator, including piston-type brake actuators, wherein the piston plate or head is sealed against an inside surface of the brake actuator housing, eliminating the requirement for a cup-shaped diaphragm, and other types of brake actuators. The brake actuator may also include an emergency chamber having a power spring as is well known in the prior art.
The improved brake actuator of this invention includes a sensor, such as a reed switch, and a generally opposed spaced magnet, which are fixed relative to the housing of the brake actuator. As will be understood from the following description of the improved vehicle brake monitor of this invention, the sensor is located relative to the magnet such that the sensor is within the magnetic field of the magnet, but is spaced from the magnet. Thus, the term xe2x80x9copposedxe2x80x9d is not intended to limit the relative location of the magnet and sensor except as required for this application. For example, where the sensor is a reed switch, the normally open reed switch sensor is closed within the magnetic field of the magnet. The vehicle brake monitor further includes a shunt having a predetermined length which is reciprocal with the brake actuator rod between the magnet and the sensor. The shunt, for example, may be a ferrous metal or ferrous alloy strip which projects radially from the rod and reciprocates with the rod between the sensor and the magnet, interrupting the signal between the sensor and the magnet when the shunt is located between the sensor and the magnet during reciprocal movement of the rod signaling at least one position of the rod relative to the housing. Although in the preferred embodiment of the vehicle brake monitor, the shunt is located in a sleeve fixed to the rod and the opposed sensor and magnet are located in a sensor sleeve surrounding the rod, the entire assembly of sensor, magnet and shunt may be separate from the rod, provided the sensor and magnet are fixed relative to the housing and the shunt reciprocates with the rod. The vehicle brake monitor of this invention may thus be utilized to indicate at least one condition of the brake actuator, such as an overstroke condition, wherein the brake actuator is nearing or exceeds an overstroke condition, or a dragging brake, wherein the brake actuator rod does not return to a xe2x80x9czero strokexe2x80x9d condition following actuation of the brakes.
In the most preferred embodiment of the brake actuator of this invention, the brake monitor includes two pairs of generally opposed space magnets and sensors, which are fixed relative to the housing, and two shunts, each having a predetermined length or position relative to the longitudinal axis of the rod, which reciprocate with the push rod between each of the opposed pairs of sensors and magnets. By utilizing two pairs of magnets, sensors and shunts, several conditions of the brake actuator may be identified and monitored, including a dragging brake, an overstroke condition, normal operation and a failure of the vehicle brake monitor for each brake actuator on the vehicle. As described more fully in application Ser. No. 09/091,057, filed Nov. 30, 1998, assigned to the assignee of this application, each brake monitor may include an RF transmitter which periodically transmits the condition of each brake actuator to a controller. Alternatively, the brake actuator monitor may be hardwired to the controller. The controller then signals the condition of each brake actuator to the vehicle operator by conventional display, such as a heads up display in the vehicle cab or a conventional display panel.
In the preferred embodiment of the vehicle brake monitor of this invention, the magnet and sensor are located in a sensor sleeve generally surrounding the brake actuator rod. In the most preferred embodiment, the sensor sleeve is a stone shield located within the brake actuator housing opening surrounding the rod of the brake actuator. In this preferred embodiment of the vehicle brake monitor of this invention, the shunt is located in a sleeve fixed to the brake actuator rod, wherein the shunt extends radially between the magnet and sensor during reciprocal movement of the sleeve on the rod. In the most preferred embodiment, the shunt sleeve on the rod of the brake actuator includes two circumferentially spaced radially extending shunts which reciprocate with the push rod between each of the pairs of sensors and magnets. Because the shunts are of different lengths and/or spaced along the longitudinal axis of the rod, the brake monitor can detect multiple positions of the rod relative to the brake actuator as described above. The most preferred embodiment of the shunt sleeve further includes a key or radially projecting rib and the sensor sleeve includes a radial slot or keyway which slideably receives the key on the shunt sleeve positioning the shunt sleeve relative to the sensor sleeve during reciprocal movement of the shunt sleeve and the brake actuator rod.
The vehicle brake monitoring system of this invention is thus relatively simple in design, rugged in construction and able to withstand the harsh environment required of brake actuators. Further, the brake actuator monitor of this invention may be utilized to monitor various conditions of the brake actuator including a dragging brake, overstroke, normal operation and a malfunctioning brake actuator monitor, particularly where the sensor is a reed switch as described above. Other advantages and meritorious features of the vehicle brake monitor of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.